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Subject[PATCH BUGFIX IMPROVEMENT 05/14] block, bfq: consider also ioprio classes in symmetry detection
Date
In asymmetric scenarios, i.e., when some bfq_queue or bfq_group needs
to be guaranteed a different bandwidth than other bfq_queues or
bfq_groups, these service guaranteed can be provided only by plugging
I/O dispatch, completely or partially, when the queue in service
remains temporarily empty. A case where asymmetry is particularly
strong is when some active bfq_queues belong to a higher-priority
class than some other active bfq_queues. Unfortunately, this important
case is not considered at all in the code for detecting asymmetric
scenarios. This commit adds the missing logic.

Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
---
block/bfq-iosched.c | 86 ++++++++++++++++++++++++---------------------
block/bfq-iosched.h | 8 +++--
block/bfq-wf2q.c | 12 +++++--
3 files changed, 59 insertions(+), 47 deletions(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index a9275ed57726..6bfbfa65610b 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -623,26 +623,6 @@ void bfq_pos_tree_add_move(struct bfq_data *bfqd, struct bfq_queue *bfqq)
bfqq->pos_root = NULL;
}

-/*
- * Tell whether there are active queues with different weights or
- * active groups.
- */
-static bool bfq_varied_queue_weights_or_active_groups(struct bfq_data *bfqd)
-{
- /*
- * For queue weights to differ, queue_weights_tree must contain
- * at least two nodes.
- */
- return (!RB_EMPTY_ROOT(&bfqd->queue_weights_tree) &&
- (bfqd->queue_weights_tree.rb_node->rb_left ||
- bfqd->queue_weights_tree.rb_node->rb_right)
-#ifdef CONFIG_BFQ_GROUP_IOSCHED
- ) ||
- (bfqd->num_groups_with_pending_reqs > 0
-#endif
- );
-}
-
/*
* The following function returns true if every queue must receive the
* same share of the throughput (this condition is used when deciding
@@ -651,25 +631,48 @@ static bool bfq_varied_queue_weights_or_active_groups(struct bfq_data *bfqd)
*
* Such a scenario occurs when:
* 1) all active queues have the same weight,
- * 2) all active groups at the same level in the groups tree have the same
- * weight,
+ * 2) all active queues belong to the same I/O-priority class,
* 3) all active groups at the same level in the groups tree have the same
+ * weight,
+ * 4) all active groups at the same level in the groups tree have the same
* number of children.
*
* Unfortunately, keeping the necessary state for evaluating exactly
* the last two symmetry sub-conditions above would be quite complex
- * and time consuming. Therefore this function evaluates, instead,
- * only the following stronger two sub-conditions, for which it is
+ * and time consuming. Therefore this function evaluates, instead,
+ * only the following stronger three sub-conditions, for which it is
* much easier to maintain the needed state:
* 1) all active queues have the same weight,
- * 2) there are no active groups.
+ * 2) all active queues belong to the same I/O-priority class,
+ * 3) there are no active groups.
* In particular, the last condition is always true if hierarchical
* support or the cgroups interface are not enabled, thus no state
* needs to be maintained in this case.
*/
static bool bfq_symmetric_scenario(struct bfq_data *bfqd)
{
- return !bfq_varied_queue_weights_or_active_groups(bfqd);
+ /*
+ * For queue weights to differ, queue_weights_tree must contain
+ * at least two nodes.
+ */
+ bool varied_queue_weights = !RB_EMPTY_ROOT(&bfqd->queue_weights_tree) &&
+ (bfqd->queue_weights_tree.rb_node->rb_left ||
+ bfqd->queue_weights_tree.rb_node->rb_right);
+
+ bool multiple_classes_busy =
+ (bfqd->busy_queues[0] && bfqd->busy_queues[1]) ||
+ (bfqd->busy_queues[0] && bfqd->busy_queues[2]) ||
+ (bfqd->busy_queues[1] && bfqd->busy_queues[2]);
+
+ /*
+ * For queue weights to differ, queue_weights_tree must contain
+ * at least two nodes.
+ */
+ return !(varied_queue_weights || multiple_classes_busy
+#ifdef BFQ_GROUP_IOSCHED_ENABLED
+ || bfqd->num_groups_with_pending_reqs > 0
+#endif
+ );
}

/*
@@ -728,15 +731,14 @@ void bfq_weights_tree_add(struct bfq_data *bfqd, struct bfq_queue *bfqq,
/*
* In the unlucky event of an allocation failure, we just
* exit. This will cause the weight of queue to not be
- * considered in bfq_varied_queue_weights_or_active_groups,
- * which, in its turn, causes the scenario to be deemed
- * wrongly symmetric in case bfqq's weight would have been
- * the only weight making the scenario asymmetric. On the
- * bright side, no unbalance will however occur when bfqq
- * becomes inactive again (the invocation of this function
- * is triggered by an activation of queue). In fact,
- * bfq_weights_tree_remove does nothing if
- * !bfqq->weight_counter.
+ * considered in bfq_symmetric_scenario, which, in its turn,
+ * causes the scenario to be deemed wrongly symmetric in case
+ * bfqq's weight would have been the only weight making the
+ * scenario asymmetric. On the bright side, no unbalance will
+ * however occur when bfqq becomes inactive again (the
+ * invocation of this function is triggered by an activation
+ * of queue). In fact, bfq_weights_tree_remove does nothing
+ * if !bfqq->weight_counter.
*/
if (unlikely(!bfqq->weight_counter))
return;
@@ -2227,7 +2229,7 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
return NULL;

/* If there is only one backlogged queue, don't search. */
- if (bfqd->busy_queues == 1)
+ if (bfq_tot_busy_queues(bfqd) == 1)
return NULL;

in_service_bfqq = bfqd->in_service_queue;
@@ -3681,7 +3683,8 @@ static bool bfq_better_to_idle(struct bfq_queue *bfqq)
* the requests already queued in the device have been served.
*/
asymmetric_scenario = (bfqq->wr_coeff > 1 &&
- bfqd->wr_busy_queues < bfqd->busy_queues) ||
+ bfqd->wr_busy_queues <
+ bfq_tot_busy_queues(bfqd)) ||
!bfq_symmetric_scenario(bfqd);

/*
@@ -3960,7 +3963,7 @@ static struct request *bfq_dispatch_rq_from_bfqq(struct bfq_data *bfqd,
* belongs to CLASS_IDLE and other queues are waiting for
* service.
*/
- if (!(bfqd->busy_queues > 1 && bfq_class_idle(bfqq)))
+ if (!(bfq_tot_busy_queues(bfqd) > 1 && bfq_class_idle(bfqq)))
goto return_rq;

bfq_bfqq_expire(bfqd, bfqq, false, BFQQE_BUDGET_EXHAUSTED);
@@ -3978,7 +3981,7 @@ static bool bfq_has_work(struct blk_mq_hw_ctx *hctx)
* most a call to dispatch for nothing
*/
return !list_empty_careful(&bfqd->dispatch) ||
- bfqd->busy_queues > 0;
+ bfq_tot_busy_queues(bfqd) > 0;
}

static struct request *__bfq_dispatch_request(struct blk_mq_hw_ctx *hctx)
@@ -4032,9 +4035,10 @@ static struct request *__bfq_dispatch_request(struct blk_mq_hw_ctx *hctx)
goto start_rq;
}

- bfq_log(bfqd, "dispatch requests: %d busy queues", bfqd->busy_queues);
+ bfq_log(bfqd, "dispatch requests: %d busy queues",
+ bfq_tot_busy_queues(bfqd));

- if (bfqd->busy_queues == 0)
+ if (bfq_tot_busy_queues(bfqd) == 0)
goto exit;

/*
diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
index 0b02bf302de0..30be669be465 100644
--- a/block/bfq-iosched.h
+++ b/block/bfq-iosched.h
@@ -501,10 +501,11 @@ struct bfq_data {
unsigned int num_groups_with_pending_reqs;

/*
- * Number of bfq_queues containing requests (including the
- * queue in service, even if it is idling).
+ * Per-class (RT, BE, IDLE) number of bfq_queues containing
+ * requests (including the queue in service, even if it is
+ * idling).
*/
- int busy_queues;
+ unsigned int busy_queues[3];
/* number of weight-raised busy @bfq_queues */
int wr_busy_queues;
/* number of queued requests */
@@ -974,6 +975,7 @@ extern struct blkcg_policy blkcg_policy_bfq;

struct bfq_group *bfq_bfqq_to_bfqg(struct bfq_queue *bfqq);
struct bfq_queue *bfq_entity_to_bfqq(struct bfq_entity *entity);
+unsigned int bfq_tot_busy_queues(struct bfq_data *bfqd);
struct bfq_service_tree *bfq_entity_service_tree(struct bfq_entity *entity);
struct bfq_entity *bfq_entity_of(struct rb_node *node);
unsigned short bfq_ioprio_to_weight(int ioprio);
diff --git a/block/bfq-wf2q.c b/block/bfq-wf2q.c
index 72adbbe975d5..ce37d709a34f 100644
--- a/block/bfq-wf2q.c
+++ b/block/bfq-wf2q.c
@@ -44,6 +44,12 @@ static unsigned int bfq_class_idx(struct bfq_entity *entity)
BFQ_DEFAULT_GRP_CLASS - 1;
}

+unsigned int bfq_tot_busy_queues(struct bfq_data *bfqd)
+{
+ return bfqd->busy_queues[0] + bfqd->busy_queues[1] +
+ bfqd->busy_queues[2];
+}
+
static struct bfq_entity *bfq_lookup_next_entity(struct bfq_sched_data *sd,
bool expiration);

@@ -1513,7 +1519,7 @@ struct bfq_queue *bfq_get_next_queue(struct bfq_data *bfqd)
struct bfq_sched_data *sd;
struct bfq_queue *bfqq;

- if (bfqd->busy_queues == 0)
+ if (bfq_tot_busy_queues(bfqd) == 0)
return NULL;

/*
@@ -1665,7 +1671,7 @@ void bfq_del_bfqq_busy(struct bfq_data *bfqd, struct bfq_queue *bfqq,

bfq_clear_bfqq_busy(bfqq);

- bfqd->busy_queues--;
+ bfqd->busy_queues[bfqq->ioprio_class - 1]--;

if (!bfqq->dispatched)
bfq_weights_tree_remove(bfqd, bfqq);
@@ -1688,7 +1694,7 @@ void bfq_add_bfqq_busy(struct bfq_data *bfqd, struct bfq_queue *bfqq)
bfq_activate_bfqq(bfqd, bfqq);

bfq_mark_bfqq_busy(bfqq);
- bfqd->busy_queues++;
+ bfqd->busy_queues[bfqq->ioprio_class - 1]++;

if (!bfqq->dispatched)
if (bfqq->wr_coeff == 1)
--
2.20.1
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